A conventional force sensor having a function of outputting a force acting in a predetermined axis direction and a torque acting around a predetermined rotation axis as electric signals is a capacitive force sensor that detects a force and a torque in accordance with changes in the capacitance values of capacitive elements, or a strain-gauge force sensor that detects a force and a torque in accordance with changes in the electric resistance value of a stain gauge. Such force sensors have been produced on a commercial basis.
A strain-gauge force sensor requires the step of attaching a strain gauge to a flexure element in the sensor manufacturing process. This complicates the assembling of the sensor. Furthermore, it is extremely difficult for a strain-gauge force sensor to contain a stopper mechanism for preventing sensor failures due to overload, and therefore, the use of such a stain-gauge force sensor is limited.
On the other hand, a capacitive force sensor has a simple sensor structure, and it is easy for a capacitive force sensor to contain a stopper mechanism for preventing sensor failures due to overload. Furthermore, a capacitive element is formed with two sets of parallel plates, and accordingly, an inexpensive force sensor can be obtained. Because of these features, capacitive force sensors are widely available on markets.
However, a capacitive force sensor detects a force in the Z-axis direction, in accordance with the sum of the capacitance values of capacitive elements. For example, such a detection method is illustrated in FIGS. 6 and 7 of Patent Document 1 filed by the applicant. In this case, the output of the sensor fluctuates due to temperature changes in the use environment, and is further affected by in-phase noise. It is of course possible to solve these problems by changing the detection circuit. However, this leads to higher force sensor production costs, which is undesirable.